[llvm] X86: add some missing lowerings for shuffles on `bf16` element type. (PR #76076)

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bjacob wrote:

@phoebewang , thanks again for the great review. It's much nicer now following your guidance, and I added a testcase. PTAL any time (this can wait until 2024!).

https://github.com/llvm/llvm-project/pull/76076